tions were made with a mural circle and a repeating circle. Arago expected to find a difference of ten or fifteen seconds, but found none. He thought that a difference no greater than one ten-thousandth would have been manifested by his observations had it existed. Arago attempted to explain his negative results by assumptions based upon the corpuscular theory of light. But Lloyd thought that the change in the length of the wave would balance the change in the direction of the ray. Arago’s observations were communicated to the Institute on December 10,1816, and excited great interest. They were quoted by Laplace and Biot. But the manuscript was mislaid and not found until 1853, when it was published. Mascart thinks that this experiment of Arago owes its reputation to Fresnel's explanation of it by his fraction.
In regard to the wave-motion involved in the transmission of light, Maxwell says: “It may be a displacement, or a rotation, or an electrical disturbance, or indeed any physical quantity which is capable o assuming negative as well as positive values. But the ӕther is loosely connected with the particles of gross matter; otherwise they would reflect more light.” Then he asks the question, “Does the ӕther pass through bodies as water through the meshes of a net which is towed by a boat?” It is difficult to obtain the relative motion of the earth and ӕther by experiment, as the light must move forward and then back again. One way is to compare the velocities of light obtained from the eclipses of Jupiter’s satellites when Jupiter is in opposite points of the ecliptic. Cornu referred, in 1883, to the difficulty of observing these eclipses, especially when Jupiter is in conjunction with the sun. On account of this difficulty observations have been neglected for the last fifty years. Observations must be made near quadratures. Cornu suggests a proper arrangement for this purpose.
At various times between 1864 and 1868, Maxwell repeated Arago's experiment in a more perfect form. A spectroscope was used, having three prisms of 60° each. A plane mirror was substituted for the slit of the collimator. The cross-wires of the observing telescope were illuminated by light reflected by a plate of thin glass placed at an angle of 45°. Light went to the mirror and was sent back to the wires from which it started after passing through six prisms. The experiment was tried when the light started in the direction of the earth’s motion, and when in the opposite; also, at different seasons of the year. In all cases the image of the wires coalesced with the wires.
Lodge states the case clearly thus: “If all the ӕther were free there would have been a displacement of the image of the wires. If all the ӕther were bound to the glass there would have been a difference on the other side. But, according to Fresnel's hypothesis there should be no difference either way. According to his hypothesis, the free ӕther, which is the portion in relative motion, has nothing to do with the refraction. It is the addition of the bound ӕther which causes the refraction, and this part is stationary relatively to the glass, and is not stream
ing through it at all. Hence the refraction is the same whether the prism be at rest or in motion through space.” Maxwell is more guarded in his own statement of the case. He says: “We can not conclude certainly that the ӕther moves with the earth, for Stokes has shown from Fresnel’s hypothesis that the relative velocities of the ӕther in the prism and that outside are inversely as the square of the index of refraction, and the deviation in this case would not be sensibly altered, the velocity of the earth being only one ten-thousandth of the velocity of light.”
In 1879, Maxwell wrote to Prof. D. P. Todd, then at the Nautical Almanac Office in Washington, asking him if he had observed an apparent retardation of the eclipses of Jupiter’s satellites depending on the geocentric position of the planet. Such observations, be thought, would furnish the only method he knew of finding the direction and velocity of the sun’s motion through the surrounding medium. In terrestrial methods of measuring the velocity of light, it returns on its path, and the velocity of the earth in relation to the ӕther would alter the whole time of passage by a quantity depending on the square of the ratio of the velocities of the earth and light, and this is quite too small to be observed.
In 1839, Babinet made a very delicate experiment on the relation of the luminiferous ӕther to the motion of the earth. He found that when two pieces of glass of equal thickness were placed across two beams of light which interfered so as to produce fringes, one of them moving in the direction of the earth’s motion and the other contrary to it, the fringes were not displaced. The experiment was made three-times by Babinet, with new apparatus each time. He concludes that here is a new condition to be fulfilled by all theories in regard to the propagation of light in refracting media. According to all the theories admitted or proposed, the displacement of the fringes should have been equal to many lengths of a fringe — that is, many millimeters — while by observation it was nothing. Stokes has calculated the result according to Fresnel’s theory, or his own modification of it, and found that the retardation expressed in time was the same as if the earth were at rest. Fizeau has pointed out a compensation in the effect of Babinet’s experiment. he says: “When two rays have a certain difference of march, this difference is altered by the reflection from the turning mirror.” By calculating the two effects in Babinet’s experiment, Fizeau finds that they have sensibly equal values, and of opposite sign.
In 1860, Angström communicated to the Royal Society of Upsala a method of determining the motion of the solar system by observations on the bands of interference produced by a glass grating. In 1863, he published the results which he had obtained. After allowing for Babinet’s correction on account of the motion of the grating, Angström finds that a difference in the direction of the observing telescope with reference to the earth’s motion might produce a displacement of the